Effect of dietary carbohydrate and phenotype on thyroid hormones and brown adipose tissue locularity in adult LA/N –cp rats

Abstract 1. Groups of lean and corpulent LA/N-cp rats were fed isoenergetic diets containing, 54% carbohydrate as maize starch (MS) or sucrose (SU), 20% protein, 16% mixed fats, plus other essential nutrients and fiber from 1.5–9 months of age. Final body weights of corpulent rats were 2–3 times those of their lean littermates, and were greater with SU than MS diet in both phenotypes. 2. Interscapular brown adipose tissue (IBAT) mass was greater in corpulent than lean and was greater with SU than MS diet in lean but not corpulent rats. IBAT cell diameters and adipocyte volumes were generally similar in both phenotypes, and were not markedly affected by dietary carbohydrate type. 3. Brown adipocyte locularity profiles were qualitatively similar in both phenotypes, and were morphologically indicative of thermogenic activity in both phenotypes. Locule profiles of corpulent animals contained a greater proportion of thermogenically less active types IV and V brown adipocytes than similarly fed lean animals, however, and locule distribution profiles were not influenced by diet. . 4. Serum T3 concentrations were similar in both phenotypes, were greater in SU than MS lean rats and were not influenced by diet in the corpulent phenotype. In contrast, serum thyroxine concentrations and percent thyroxine uptake were not influenced by diet or phenotype. 5. These results are consistent with a partial impairment in BAT-mediated thermogenic activity in the corpulent phenotype and suggest that obesity in this strain may be due to factors other than biochemically defective brown adipose tissue thermogenesis.

摘要
1. 将瘦型与肥胖型LA/N-cp大鼠分为四组，自1.5月龄至9月龄期间饲喂等热量饲料：两种饲料的碳水化合物占比均为54%，分别以玉米淀粉（maize starch, MS）或蔗糖（sucrose, SU）作为碳水来源，同时添加20%蛋白质、16%混合脂肪及其他必需营养素与膳食纤维。实验结果显示，肥胖型大鼠的最终体质量为其瘦型同窝仔鼠的2~3倍；且两种表型的大鼠在饲喂蔗糖饲料时的体质量均高于饲喂玉米淀粉饲料的同组大鼠。
2. 肥胖型大鼠的肩胛间棕色脂肪组织（interscapular brown adipose tissue, IBAT）质量高于瘦型大鼠；且在瘦型大鼠中，饲喂蔗糖饲料的IBAT质量显著高于玉米淀粉饲料组，但该差异并未在肥胖型大鼠中观测到。两种表型大鼠的IBAT细胞直径与脂肪细胞体积总体相近，且饲料碳水化合物类型对上述指标无显著影响。
3. 两种表型大鼠的棕色脂肪细胞脂泡分布特征在定性层面相似，二者的形态学特征均提示存在产热活性。但肥胖型动物的脂泡分布中，产热活性较低的IV型与V型棕色脂肪细胞占比，高于相同饲料饲喂条件下的瘦型动物；而饲料类型对棕色脂肪细胞的脂泡分布特征无显著影响。
4. 两种表型大鼠的血清三碘甲状腺原氨酸（T3）浓度相近；瘦型大鼠中，饲喂蔗糖饲料的血清T3浓度高于玉米淀粉饲料组，但该效应未在肥胖型大鼠中出现。与之相反，血清甲状腺素浓度与甲状腺素摄取百分比不受饲料类型或大鼠表型的影响。
5. 上述实验结果与肥胖型表型中棕色脂肪组织介导的产热活性存在部分受损的假说相符，且提示该品系大鼠的肥胖可能并非源于生化层面的棕色脂肪组织产热功能缺陷。